The present invention pertains generally to navigation systems for airborne platforms such as aircraft. More particularly, the present invention pertains to pulse Doppler radars and radar altimeters that can be operationally combined to establish an effective airborne navigation system. The present invention is particularly, but not exclusively, useful as a back-up system for a GPS navigational system, and for stand-alone use in specific applications such as terrain avoidance.
GPS (i.e. a Global Positioning System) is a satellite-based radio navigation, positioning and time transfer system that provides highly accurate navigational information on a continuous global basis to an unlimited number of properly-equipped users. Important aspects of the system are that GPS is unaffected by weather, and it provides a worldwide common grid reference system that is based on an earth-fixed coordinate system. Nevertheless, despite these beneficial aspects, GPS is susceptible to system outages, and it can be jammed.
For flight missions that require accurate position identification and precise navigational information for an airborne platform (e.g. an aircraft), GPS is an extremely effective tool. Due to the susceptibilities noted above, however, GPS should not be relied upon as a stand-alone navigational system. Stated differently, redundancy is a desirable attribute for any airborne navigational system. With this in mind, several advantages for having a back-up navigational system for GPS can be considered. First, the back-up system should be able to effectively assume the role of the primary system (i.e. GPS) when the latter becomes inoperative or inoperable. Second, and perhaps equally important, a back-up system can be used to verify the operation of the primary system. Third, a back-up system can be particularly valuable if it is also able to provide additional information, such as terrain avoidance actions, that may not otherwise be provided by the primary navigation system.
An important attribute for a back-up navigational system is that it be able to operate independently of the primary system it is intended to support. Preferably, such a back-up system not only operates independently of the primary system, it also relies on different physical phenomena for its functionality. This latter attribute is particularly important when jamming of the primary system (e.g. GPS) is a distinct possibility. Vis-a-vis GPS, a Doppler navigation system is an attractive candidate for use as a back-up to an airborne GPS system for several reasons.
Unlike an airborne GPS system, which requires external communication with earth orbiting satellites, a Doppler navigation system can be entirely contained inside the airborne platform (aircraft). Thus, a Doppler system is not particularly vulnerable to radio interference or hostile jamming. Also, unlike an airborne GPS system, a Doppler system need not be an xe2x80x9call or nothing at allxe2x80x9d system. This is so because a Doppler system can easily include several functional components that are each capable of independent operation. Thus, such a system has several inherent advantages. For one there can be a built-in redundancy that makes operation possible, even though some components of the system are inoperative or inoperable. Further, the use of independently operating components allows the system to be specifically tailored for the particular needs of the system application. Also, as is well known, in a Doppler system each individual radar beam is capable of providing both distance (time delay) and speed (frequency shift) information. Thus, similar information from different geometric perspectives can be combined to provide additional information.
In light of the above it is an object of the present invention to provide a system for identifying the position of an airborne platform that can serve as a precise and accurate back-up navigational system for a GPS system. Another object of the present invention is to provide a system for identifying the position of an airborne platform that can function as a stand-alone system using only internal, platform-mounted equipment. Still another object of the present invention is to provide a system for identifying the position of an airborne platform that, in addition to an airborne navigation mission, is also capable of performing specialty missions such as terrain avoidance. Yet another object of the present invention is to provide a system for identifying the position of an airborne platform that is relatively easy to manufacture, is simple to use, and is comparatively cost effective.
A radar system for identifying the position of an airborne platform on a flight path is mounted on the platform to direct an xe2x80x9cnxe2x80x9d number of radar beams on respective predetermined paths toward the surface of the earth. The intent here is to generate an xe2x80x9cnxe2x80x9d number of return signals. For the present invention, xe2x80x9cnxe2x80x9d will generally be greater than or equal to three, and each return signal will contain information about the distance and speed of the platform relative to a point on the surface of the earth. The system also includes a computer that uses the xe2x80x9cnxe2x80x9d number of return signals to compute a ground speed, a direction of flight, and an altitude for the platform. These individual bits of information, together with information about the start point of the platform, are then collectively used to identify the position of the platform on its flight path.
For one embodiment of the present invention the system uses a single radar which sequentially directs its radar beam along the xe2x80x9cnxe2x80x9d predetermined beam paths. In another embodiment, a separate radar transceiver is used for each of the xe2x80x9cnxe2x80x9d beams. For both embodiments, one of the radar beam paths is preferably oriented in a direction that is forward of the platform (aircraft) and downward at an angle xcex1 from the flight path of the platform (aircraft). This same beam path is also directed laterally from the flight path of the platform at a positive angle xcex2. Another radar beam is similarly oriented, but it is directed laterally at a negative angle xcex2 from the flight path of the platform. For this second embodiment, still another transceiver directs a radar beam along a beam path that is substantially perpendicular to the earth""s surface.
With the arrangement of the radar beam paths disclosed above, the (at least three) return signals can be used by the computer using well known mathematical techniques to solve for the ground speed of the platform (aircraft), its altitude, and any change in the direction of flight for the platform. Together, this information can then be combined and continuously updated to identify the position of the platform on its flight path, relative to a known start point.
A start point for the platform can be established in either of two ways. When the system of the present invention is being used as a back-up system for a GPS navigational system, the last computed GPS position can be used as the start point. Alternatively, the start point can be established using a site-specific radar clutter model. As intended for the present invention, such a site-specific radar clutter model could be based on National Imagery and Mapping Agency (NIMA) Digital Terrain Elevation Data (DTED) and NIMA Digital Feature Analysis Data (DFAD). Additionally, and regardless whether an operative GPS navigational system is being used, it is envisioned for the present invention that the radar return signals will be matched with the site-specific radar clutter model. This is to be done for at least two reasons. First, the resultant information can be effectively used to generate a terrain-avoidance alarm. Second, the resultant information can be used to verify the proper operation of the GPS system. Further, even when the site-specific clutter model or a last GPS position is not known, it is to be appreciated that the system can still be used to determine the slope of the surface of the earth relative to the flight path. Importantly, the system is able to use this slope information to indicate a preferred flight path for terrain avoidance. As implied, this can be done even though the exact position of the aircraft may not be known.